In pharmacological research on hypertension, recent attention has focused on the study of the renin-angiotensin-aldosterone system, and, in particular, on the development of an effective anti-hypertensive agent which would, theoretically, achieve its result by inhibiting the action of angiotensin converting enzyme in converting antiogensin I to angiotensin II. The inhibition of the production of angiotensin II became important because of the discoveries of that angiotensin II is the most potent pressor agent (vasoconstrictor) present in the mammalian body and, in addition, stimulates the adrenal cortex to release aldosterone, thereby causing excessive sodium retention and fluid retention, contributing further to the hypertensive state. Thus, inhibiting the conversion of angiotensin I to angiotensin II is believed to work directly on the primary biochemical mechanisms creating increased blood pressure. For a description of the mechanisms and of the mammalian renal-angiotensin-aldosterone system, see Hypertension, Genest et al., ed., Chapters 6.1, 6.2, 7.1, 7.2, and 7.3 (McGraw Hill, 1977) and John H. Laragh, "The Renin System in High Blood Pressure, From Disbelief to Reality: Converting-Enzyme Blockade for Analysis and Treatment", Prog. in Cardio. Vasc. Disease, XXI, No. 3, 159-166 (November, 1978).
An extensive list of angiotensin converting enzyme inhibitors is set forth in Suzanne Oparil's article entitled "Angiotensin I Converting Enzyme and Inhibitors" in Genest et al., supra, Chapter 6.3, at pp. 159-161. These inhibitors are summarized in Table I, p. 161, thereof and include chelating agents, sulfonylating agents, heavy metal ions, sulfhydryl binding reagents, and various peptides. The polypeptides described therein as angiotensin converting enzyme inhibitors include hormones, such as bradykinin; products of substrate digestion such as His-Leu, Phe-Arg, and Arg-Pro-Pro; and various snake venom polypeptide extracts. Two of the most potent and most studied inhibitors are the Bothrops jararaca snake venom extract, the pentapeptide (Pyr-Lys-Trp-Ala-Pro), also referred to as BPP.sub.5a, and the nonapeptide (Pyr-Trp-Pro-Arg-Pro-Gln-Ile-Pro-Pro), also referred to as BPP.sub.9a. (BPP stands for Bradykinin Potentiating Peptide). BPP.sub.9a has been shown to be an effective anti-hypertensive agent in clinical studies on humans with certain forms of hypertension. However, BPP.sub.9a is not orally active as an anti-hypertensive agent. For a summary of the clinical aspects of BPP.sub.9a see Genest et al., supra, Chapter 6.3, pp. 163-4.
More recently, a series of proline derivatives has been found to be significantly more potent as inhibitors of angiotensin converting enzyme and as anti-hypertensive agents than BPP.sub.9a. Of these proline derivatives, D-3-mercapto-2-methylpropanoyl-L-proline has been reported to be the most effective, including being effective when administered orally. These proline and mercaptoproline derivatives and various pharmacological test results thereon are described in Cushman et al., "Design of New Anti-hypertensive Drugs: Potent and Specific Inhibitors of Angiotensin Converting Enzyme", Prog. in Cardio. Diseases, Vol. XXI, No. 3 (Nov./Dec., 1978), and in U.S. Pat. Nos. 4,046,889 and 4,105,776, both to Ondetti and Cushman.